Layers and thin films of crystalline materials are important in a wide number of applications including glazing, electronics and photovoltaics. It is often important that such layers are smooth and free of holes or defects. Furthermore, it is desirable to be able to produce such layers quickly, easily and with minimal expense. Layers and thin films are typically produced by vapour deposition or solution deposition. Both processes can have inherent difficulties.
Solution processing is a promising technology for production of layers of crystalline materials. As compared to vapour deposition, solution processing is generally applicable to large-scale substrates, easily transferable to reel-to-reel printing and cheaper than vacuum techniques. Well-known solution based techniques include graveur coating, slot dye coating, screen printing, ink jet printing, doctor blade coating, spray coating and spin-coating. However, there are still problems associated with known solution coating techniques and the precursor solutions used in these solution based techniques. In particular, conventional precursor solutions used in solution processing of crystalline materials often require long anneal times for film formation. This both slows the production of the layers of crystalline materials and also increases the associated costs due to greater energy requirements. Furthermore, films produced by solution coating often have surfaces which are too rough for practical application to thin film optoelectronic devices.
One area in which the efficient and effective production of thin films of crystalline materials is of great importance is optoelectronics and photovoltaics. For instance, perovskite solar cells, which have demonstrated great promise for photovoltaic technology with the lowest cost and highest efficiency, often require high quality films of the crystalline materials perovskite. Through rational device architecture design, materials interface engineering, as well as processing technique optimization, a recorded efficiency around 18% has been attained for organic-inorganic metal halide perovskites, showing great potential for commercialization to compete with traditional crystalline silicon solar cells. Although the device performance of perovskite solar cells has improved at an unprecedented rate over recent years, the basic properties of organic-inorganic metal halide perovskites, for instance CH3NH3PbX3 where X═Cl, Br, I (which is also referred to herein as MAPbX3), such as the role of cation and anion, are still not well understood. Most research focuses on tuning the perovskite band gap by changing the ratio of either anions (Br− to I−) or cations (formamidinium (FA) to methylammonium (MA)). Known methods for producing layers of organic-inorganic metal halide perovskite materials typically comprise solution- or vacuum-processing a metal halide and a halide salt of the organic component, for instance PbI2 and CH3NH3I (MAI).
It has become apparent that the quality and form of the film of perovskite used in organic-inorganic metal halide perovskite solar cells is of great importance for device efficiency. There is a need for a new approach to form high quality, smooth and substantially pinhole-free thin films of crystalline materials. It is also desirable to provide an efficient and scalable process by which to produce such films.